Recently, in the optical pickup apparatus, the wavelength-shortening of the laser light source used as the light source for reproducing of the information recorded in an optical disk or recording of the information in the optical disk is advanced. For example, a laser light source of wavelength 405 nm such as a blue-violet semiconductor laser, or a blue-violet SHG laser which conducts the wavelength conversion of the infrared semiconductor laser by using the second harmonic wave generation is putting to a practical use.
When these blue-violet laser light sources are used, in the case where an objective lens of the same numerical aperture (NA) as DVD (Digital Versatile Disk) is used, the information of 15-20 GB can be recorded in an optical disk of diameter 12 cm, and in the case where NA of the objective lens is increased to 0.85, the information of 23-25 GB can be recorded in the optical disk of diameter 12 cm. Hereinafter, in the present specification, the optical disk and photo-magnetic disk for which the blue-violet laser light source is used, are generally referred as “high density optical disk”.
Hereupon, only by saying that the information can be adequately recorded and/or reproduced for such a high density optical disk, it is difficult to be said that a value as a product of the optical disk player and/or recorder is enough. In the present time, DVD or CD (Compact Disc) in which various information are recorded is put in a market. When the actuality is based on, by only a case where the information can be recorded and/or reproduced for the high density optical disk, it is insufficient, and for example, a fact that the information can be adequately recorded and/or reproduced in the same manner also for a user-own DVD or CD, introduces to a fact that a commercial value as the optical disk player and/or recorder is increased. For such a background, it is desirable that the optical pickup apparatus mounted in the optical disk player/recorder for the high density optical disk has a performance by which the information can be adequately recorded and/or reproduced while the compatibility is being kept with also any one of 3 kinds of optical disks of the high density optical disk, DVD and CD.
As a method by which the information can be adequately recorded and/or reproduced while the compatibility is being kept with also any one of the high density optical disk and DVD, furthermore, CD, a method by which an optical system for the high density optical disk and an optical system for DVD or CD are selectively switched corresponding to the recording density of the optical disk for which the information is recorded and/or reproduced, can be considered. However, because a plurality of optical systems are necessary for the method, it is disadvantageous for down-sizing, further, the cost is increased.
Accordingly, for the purpose to intend that the structure of the optical pickup apparatus is simplified and the cost is reduced, it can be said that even in the optical pickup apparatus having the compatibility, it is preferable that the common optical system for the high density optical disk and for DVD or CD is used and the number of parts structuring the optical pickup apparatus are reduced at most. Further, it is most advantageous that the common objective optical system arranged in opposite to the optical disk is used with each other, in the simplification of the structure of the optical pickup apparatus and the cost reduction.
Hereupon, in the case where it is intended to realize the compatibility by using the common objective optical system in the optical pickup apparatus, the light source wavelengths or the protective substrate thicknesses used for respective optical disks are different. Therefore, it is necessary that any scheme for forming a light converged spot whose aberration is finely corrected on the information recording surface of the optical disk.
As a mode of the aberration correction, it is considered that a divergence angle of the light flux entering into the objective optical element is changed. According to a mode of such an aberration correction, there is a problem that, the off-axis performance is worsened (i.e. the coma is more largely generated when the lens shift is conducted at the time of the tracking, as larger the degree of divergence is) corresponding to the degree of divergence of the light flux entering into the objective optical element, and it is not preferable.
As another mode of aberration correction, a diffractive structure giving the diffractive action is provided on the optical surface of the objective optical element (for example, refer to Patent Document 1).
(Patent Document 1) Tokkai No. 2002-298422
However, according to such a conventional technology, when the light fluxes entering into the objective optical element have 2 different wavelengths, the spherical aberration correction can be finely conducted also for any light flux, however, for the light fluxes of 3 different wavelengths, it is difficult that the spherical aberration correction can be finely conducted also for any light flux.
When more specifically described, for example, the wavelengths used for the high density optical disk, DVD and CD, are respectively, λ1 is about 400 nm, λ2 is about 655 nm, and λ3 is about 785 nm. Herein, because λ1:λ3≈1:2, in the blaze shaped diffractive structure written in Patent Document 1, a ratio of diffraction order in which the diffraction efficiency is the maximum, is λ1:λ3=2:1. For example, when λ1 is the 6-th order, λ3 is the 3-rd order. Further, because the effect of the diffraction is determined by the wavelength×the difference of diffraction order, and a pitch of diffractive ring-shaped zone. When a ratio of diffraction orders of the wavelengths λ1 and λ3 is 2:1, a value of λ1×2−λ3×1 is decreased. For example, when the diffractive structure is designed as the blaze wavelength is a value close to the even number times of the wavelength λ1, the mutual diffractive action in the light flux of the wavelength λ1 and the light flux of the wavelength λ3 is decreased, and it becomes difficult that the recording and/or reproducing of the information is respectively conducted on the high density optical disk and CD by using the same objective optical element.
On the one hand, even when the difference of the wavelength×diffraction order is small, it is theoretically possible that the compatibility can be attained when a small diffractive action is used. However, in this case, it is necessary that a pitch of the diffractive ring-shaped zone is decreased. Thereby, problems are generated in which the light amount is lowered, manufacturing of the lens becomes difficult, and the aberration is largely generated due to the wavelength variation within the minute range of several nm by the output change of the laser light source.